Measurements of Signal-to-Interference Ratios play an important role in the control of many aspects of a mobile radio communications system. For example, in a communications system operating according to the WCDMA technology, a radio base station regularly measures the Signal-to-Interference Ratio (SIR) of signals received from a mobile station (MS) in order to be able to provide the MS with adequate power control commands. Since WCDMA uplink channels from different MSs are non-orthogonal, signals from different MSs will interfere with each other, and unless power control is employed, the inter-user interference will cause the transmissions from some MSs to drown in the transmissions from other MSs. The transmission conditions for an MS change at a short time scale as the MS and/or other objects along the transmission path move. Hence, a fast uplink power control mechanism is essential. Often, a SIR estimate used for uplink power control is generated as soon as an uplink signal has been received, before the signal is decoded. The SIR estimate is then compared to a target SIR, and a power control command is sent to the MS in accordance with the result of this comparison.
Since WCDMA uplink channels are non-orthogonal, the interference in a cell is a limiting factor, which determines the amount of uplink data that can be transmitted in the cell. In order to increase the capacity of a cell, interference cancellation mechanisms have been introduced in some communications systems. When interference cancellation is employed, interference from other channels is removed from the received signal prior to decoding, and hence, a higher interference level can be tolerated before a signal, received at a certain received signal power, becomes non-decodable.
As the interference cancellation allows for decoding of a received signal at higher interference levels, the received signal power measurements used for the SIR estimation will have to be performed at increased interference levels. This leads to a decreased accuracy of the received signal power measurements, and thus to a reduced efficiency of the uplink power control.